The best known commerical composition for coating a surface to make it retroreflective is described in Palmquist et al. U.S. Pat. No. 2,963,378. This composition comprises a liquid vehicle and a dispersion of transparent glass micro- spheres, each of which carries a hemispherical "cap" of vapor- coated aluminum. When the composition is coated onto a surface, a percentage of the microspheres become oriented with their uncapped surface disposed toward the outer surface of the coating. Light rays incident on the coating are approximately focused by these oriented microspheres onto their respective hemispherical caps, whereupon the light rays are reflected along substantially the same axis as they traveled to the microspheres. The result is a bright retroreflection of light.
While such coating compositions have been quite successful, they do not have a maximum retroreflectivity, since statistically only about one-third of the microspheres in a final coating are oriented to give maximum retroreflectivity. Further, such compositions are generally limited to a metallic color, caused by the hemispherical coating of aluminum. And when coating made from the compositions are wet, as from rain or condensation of moisture, their retroreflective properties are greatly reduced.
These deficiencies have been recognized during the approximately 20 years that coating compositions as described have been marketed. But despite a desire for improvement, no one insofar as known, ever provided a coating composition that countered the described deficiencies.